By chance and not planed, photographing with a Holga 120 GN with a 6x4,5 mask, I centered the Sun at the edge of the upper side of the mask. My first thought was, «Oh, I have a light leak there. But looking closer I could see what happened. This was the first time I noticed such rings in the not directly exposed part of the film. The most curious is that the entire film was exposed at the B position, unwanted and is overexposed, the image is very white.
I did an enlargement of the rings as shown below:
Well, I thought that perahps many of you never saw this effect, like me now, so I decided to make a post on it.
This happened when I was exposing a film to be developed in the not so successful new Caffenol 5555, the film was also overdeveloped but this issue could be repaired by bleaching and refixing the film. The margins are now transparent and clean like they should be from the beginning. But the film was completely black after first development.
Whether this phenomenum is physical or chemical, I don't know. I would say it is physical and has to do with the difraction of the light passing a sharp edge.